Modern colour printing is a high quality business, and more printing companies are introducing testing to ensure that what they print is what the customer requires. This has lead to the increase in the demand for accurate test equipment.
Changes in the types and uses of technology within the printing industry have meant that the available techniques for testing have outstripped existing test equipment. This has lead to the requirement for a new type of test instrument.
Until approximately 5 years ago, and still with some of the smaller printing companies, the following methods were used.
An original artwork is digitised by a device called a scanner to obtain a transparency or picture and, via optical and electronic techniques, an image is produced in a computer readable digital form. The digital image is then manipulated by a computer system and, once completer converted into a form that can be projected onto a piece of film. The film contains the four basic printing colours (Cyan, Magenta, Yellow and Black) and is imaged and developed. The next stage is to make printing plates (one for each colour) from the film which plates are then used to produce the printed picture.
The film is produced by a device called an imagesetter, which uses a laser to mark the film ready for processing; this creates "dots" that make up a printed image. Control of the film production process is achieved by measuring the size and spacing of the dots created on the film and feeding this information back to the imagesetter. Thus, if an area that the imagesetter believes to be 50% is measured at 46%, the measured data is entered into the imagesetter. It will then adjust the exposure level of the laser to compensate. Current test instruments for this technique are called densitometers and they measure one criteria, the density of the printed area, and calculate the percentage dot area. These instruments work by shining a light at the film (or a test proof on paper) and measuring the light transmitted by the film (or reflected by the test proof). The amount of light transmitted/reflected signifies the density of the area being tested.
The goal for some years has been to eliminate the film stage and image directly onto the printing plates, thereby eliminating the associated costs, time and variations introduced through the number of processes involved. Methods to do this have been available in recent years, but the ability to measure accurately the imaged plate has been a problem, with the result that printers are loath to make use of these methods.
The method of correction of errors in dot size/spacing on a printing plate are the same as for the film, with an imagesetter producing the plate in the same way as film. However, as there is no satisfactory method of measuring the dot size/spacing on a printing plate, it is not possible to enter the actual data to allow compensation. Several manufacturers of densitometers have tried to use existing approaches to measure plates. However, with the grained aluminium plates used almost universally today, conventional densitometers are not suitable because of the immense amount of light scatter that causes inaccurate (and inconsistent) readings. This lack of accurate and reliable measuring equipment is the major reason that direct to plate technology has not been taken up widely by the printing industry.